The present invention relates to liquid crystal display apparatuses which require a transition of liquid crystal molecules in a liquid crystal layer from an initial alignment to a predetermined displayable alignment in advance of displaying. More particularly, this invention relates to an improvement in methods of driving such apparatuses with the transition of the alignment of liquid crystal molecules.
Various kinds of liquid crystal display apparatuses have conventionally been proposed and practicalized. In recent years, liquid crystal televisions have been expected to spread.
Widely used twisted nematic mode liquid crystal display apparatuses which employ nematic liquid crystals have shortcomings such as a slow response and a narrow viewing angle.
In-plane switching mode liquid crystal display apparatuses having a wide viewing angle have shortcomings in response speed and aperture ratio. Ferroelectric liquid crystal mode liquid crystal display apparatuses hereinafter referred to as “FLC type liquid crystal display apparatuses”) exhibit a quick response and have a wide viewing angle, but also have great shortcomings in shock resistance, temperature property and the like.
In contrast, optically compensated bend mode (or optically compensated birefringence mode) liquid crystal display apparatuses (hereinafter, referred to as “OCB mode liquid crystal display apparatuses”), which were proposed in Japanese Unexamined Patent Publications No. Hei 7-84254 or Shingaku Technical Reports (EDI98–144, on page 19, published by the Corporation of Electricity Communication Institute) exhibit a quick response and have a wide viewing angle. The apparatuses, therefore, will be expected to be applied to liquid crystal televisions and the like as transmission or reflection type liquid crystal display apparatuses hereafter.
FIG. 7a shows an example of OCB mode liquid crystal display apparatuses. A liquid crystal panel 2 includes an array substrate 3a with transparent pixel electrodes 4a provided thereon, a counter substrate 3b with transparent counter electrodes 4b provided thereon and a liquid crystal layer 7 sandwiched between the array substrate 3a and the counter substrate 3b. Liquid crystal alignment layers 6a and 6b of polyimide are formed on internal surfaces of the array substrate 3a and the counter substrate 3b, on which the pixel electrodes 4a and the counter electrodes 4b are provided, respectively. Both the liquid crystal alignment layers 6a and 6b had been treated with rubbing, and the substrates 3a and 3b are disposed so that the rubbing directions of the liquid crystal alignment layers 6a and 6b are parallel to each other. The liquid crystal layer 7 is filled with a nematic liquid crystal material having a positive dielectric anisotropy.
When a voltage is not applied between the pixel electrode 4a and the counter electrode 4b, pretilt angles of liquid crystal molecules 7a on both of the array substrate 3a and the counter substrate 3b are approximately several to 10 degrees in reverse directions. As shown in FIG. 7a, the liquid crystal molecules 7a offers an alignment such as to be inclined symmetrically and outwardly on a plane or a spray alignment.
In a case of an OCB mode liquid crystal display apparatus, by applying a voltage pulse (hereinafter, referred to as “a voltage pulse for transition”) with a comparatively high voltage between the pixel electrode 4a and the counter electrode 4b in a short time when a main power is switched on, for example, the liquid crystal molecules 7a having a spray alignment as shown in FIG. 7a locally rise and a micro area of bend alignment or bend alignment including twisted alignment (hereinafter, referred to as a “transition core”) is generated as shown in FIG. 7b. The transition core is spread by repeatedly applying the voltage pulse for transition. The transition of the whole liquid crystal material in the liquid crystal layer 7 to bend alignment enables an OCB mode liquid crystal display apparatus to display. An OCB mode liquid crystal display apparatus can display by utilizing a change in retardation resulting from a change in bend alignment of the liquid crystal molecule 7a, which is caused by applying display signal driving voltage.
On the external surface of the liquid crystal panel 2, there is disposed a phase-compensation plate 8 for optically compensating in order to enable a low voltage driving of the liquid crystal panel 2 as well as widen a viewing angle, with an optical axis thereof being fixed in a predetermined direction.
In a case of an OCB mode liquid crystal display apparatus, an inducement of such initially transition from spray alignment to bend alignment and a completion of the transition in the whole pixel area of the liquid crystal panel 2 in a short time are required before turning into an ordinary display-driving mode, as described above.
An FLC type liquid crystal display apparatus and a phase transition type liquid crystal display apparatus involve similar requirements before being shifted to an ordinary display-driving mode.
These liquid crystal display apparatuses have the following problems.
If the transition of the liquid crystal molecules to a displayable alignment is not carried out sufficiently, a fine display can not obtained when shifted to a display-driving mode. For example, in an OCB mode liquid crystal display apparatus, when the transition to bend alignment is not completed and an area of spray alignment remains locally, the remaining area becomes a bright point in display driving and looks like a point defect. The image is displayed dimly and unstably for several seconds to several minutes after starting display driving. Consequently, the transition to bend alignment must be certainly completed before being shifted to the display-driving mode. The transition core, however, is accidentally generated or not generated in the same place even if the voltage pulse for transition is applied on the same conditions, leading to the difficulty in certainly completing the transition in a short time.
Generally, a backlight is turned on when a main power of the apparatus is switched on. In liquid crystal televisions, an output of voice from a speaker starts simultaneously. However, in liquid crystal display apparatuses for carrying out the transition of the alignment of liquid crystal molecules in the liquid crystal layer to a predetermined alignment in advance of displaying, it occasionally takes a long time to be shifted to the ordinary display-driving mode. It is a waste of energy to switch on the backlight for the shift period to the display-driving mode or the transition period. The display having many point alignment defects and plane alignment defects due to pixels with no transition or under transition as well as the blinking of the whole screen due to the application of the voltage pulse for transition are the causes of users' discomfort and anxiety. Disappearance of the above-mentioned defects due to alignment transition can delete such sense of incongruity thereby realizing liquid crystal display apparatuses which can display images with high quality and are excellent in commercial view.
Operations of liquid crystal display apparatuses must be assured in a wide temperature range for being applied to various uses. Further, in an OCB mode liquid crystal display apparatus, the transition to bend alignment must be certainly completed in a short time in such wide temperature range for assuring the operations.
In a 10-inch model active matrix type liquid crystal display apparatus, for example, the transition can be completed in a short time of 0.5 to 1 second at room temperatures of around 25° C., while it occasionally takes along time, several minutes depending on conditions, for the whole liquid crystal layer to carry the transition to bend alignment at a low temperature in a range of −10 to 0° C. That is, the OCB mode liquid crystal display apparatus is required to certainly complete the transition for displaying in a short time, several seconds at the longest in a wide temperature range as in a case of a general display apparatus.